[m-rev.] for review: delete see/seen/tell/told from io.m

[Zoltan Somogyi]

In a thread on m-dev starting on 2021 august 23, we agreed to break up
io.m, which is by far the biggest module in the system, after the next release.
Since the release has now branched off, this diff implements the first step.

My proposals for what should be done with the rest of io.m, based mostly
on the discussion then:

Set A: We could move see/seen/tell/told predicates to prolog.m.
DONE

Set B: The predicates involving the reporting of offsets within files,
and seeking to them. They are not needed in most cases, but I don't think
it is worth moving them elsewhere.
PROPOSAL (from julien): move them to wherever binary input/output preds end up

Set C: The predicates that open/read/write/close binary streams.
I am slightly in favour of keeping these in io.m.
PROPOSAL (from julien): move to four submodules, read/write x text/binary
PROPOSAL: leave this decision until other moves done (when we know io.m's updated size)

Set D: I think we should move the predicates that read and write bitmaps
to bitmap.m.
PROPOSAL: move the code to bitmap.m now
PROPOSAL (from julien): move the code to write arrays to array.m now; agreed
PROPOSAL: leave io.write_list in io.m for now

Set E: We could carve off the input_stream_fold* predicates. I am neutral
about this.
PROPOSAL: leave this decision until other moves done

Set F: We could move everything in the "file handling predicates" section
to a new module, maybe file.m or files.m. I think this would be good idea.
PROPOSAL: move these to io.file.m now

Set G: We would move the predicates handling temp files, either to a separate
temp_files.m module, or to file.m/files.m. I am in favour of the latter.
PROPOSAL: move these to io.file.m now

Set H: We could carve off progname and command_line_args. I don't think
this is worth it.
PROPOSAL: leave in io.m

Set I: We should move the predicates handling environment vars to
environment.m, as discussed.
PROPOSAL: move them to io.environment.m now

Set J: We could move the call_system* predicates to a new module, maybe
call_system.m. I am neutral on this.
PROPOSAL: move them to io.call_system.m now

Set K: We could move the predicates operating in the globals to a new
module globals.m, if we didn't mind the name clash with compiler/globals.m.
PROPOSAL: move them to io.globals.m now

Set L: We should move the predicates printing stats to benchmarking.m
(those that aren't there already).
PROPOSAL: move them to benchmarking.m now

When moving predicates from io.m to a new module (which I propose
to do one or two of the above sets at a time), I intend to

- move the code,
- fix whatever breaks in the rest of the system,
- add forwarding predicates from io.m to the predicates' new home,
  marking them as obsolete.

Adding the forwarding predicates last front-loads the task of updating
code for me/us, while letting other users of Mercury choose when to
switch.

Opinions?

Zoltan.
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